Describe the histology of cartilage and bone. Add a note on the structure of blood vessels.

Cartilage

• Avascular, aneural and alymphatic, nourished by diffusion from the perichondrium or from synovial fluid
• Matrix contains type II collagen and proteoglycans rich in chondroitin sulphate, which stains basophilic
• Cells near the surface are flattened and young; those deeper are rounded and lie in isogenous groups (cell nests) from interstitial growth

Bone

• Cells — osteoprogenitor, osteoblast (forms matrix), osteocyte (in a lacuna, communicating through canaliculi), osteoclast (multinucleate, resorbs bone in a howship lacuna)
• Matrix — type I collagen with hydroxyapatite; osteocalcin, osteonectin
• Osteoclasts derive from the monocyte–macrophage line, unlike the other three, which are mesenchymal

Blood Vessels

• Capillary types — continuous (muscle, brain), fenestrated (glomerulus, gut, endocrine glands), sinusoidal (liver, spleen, bone marrow) with the widest lumen and largest gaps

Growth of Cartilage and Bone

• Bone can grow only by apposition, because its mineralised matrix cannot expand from within — which is why growth in length depends entirely on the cartilaginous epiphyseal plate
• Bone is remodelled throughout life; osteoclasts cut a tunnel and osteoblasts line it with concentric lamellae, forming a new osteon

Applied Aspects

• Articular cartilage cannot regenerate, being avascular and without a perichondrium; damage is repaired by mechanically inferior fibrocartilage, which is the basis of osteoarthritis
• Osteoporosis affects cancellous bone first — hence fractures of the vertebrae, neck of femur and distal radius, all sites rich in trabeculae
• Bone is remodelled continuously, about 10% of the skeleton a year; Haversian systems are cut out and rebuilt, leaving interstitial lamellae as their remnants
• Atherosclerosis begins in the intima of elastic and large muscular arteries; the plaque narrows the lumen and may rupture and thrombose
• Aortic dissection follows a tear in the intima with blood tracking into the diseased media — common in Marfan syndrome and hypertension
• Sinusoidal capillaries in the liver and marrow permit cells and large molecules to pass, which is why these organs are common sites of metastasis
• The fenestrated capillary of the glomerulus, with its thick basement membrane and podocyte slit diaphragms, forms the filtration barrier; its damage causes proteinuria
• Varicose veins show hypertrophy then atrophy of the media with fibrosis, and incompetent valves
• Rickets and osteomalacia show wide osteoid seams on undecalcified sections — matrix is laid down but not mineralised
• Bone marrow biopsy is taken from the posterior iliac crest, where haemopoietic marrow persists throughout life
• Cartilage grafts survive well because they are avascular and provoke little immune reaction — used in rhinoplasty and ear reconstruction
• Costal cartilage calcifies with age and becomes visible on a chest radiograph, a change used in estimating age
• Endothelium is not a passive lining — it secretes nitric oxide, prostacyclin and endothelin, and its dysfunction is the first step in atherosclerosis
• Chondrosarcoma and osteosarcoma are the malignant tumours of cartilage and bone; osteosarcoma typically arises at the metaphysis of the knee in adolescence
• Bone is a store of calcium as well as a structure, and the two roles compete — sustained hyperparathyroidism sacrifices the skeleton to maintain the serum calcium
• Decalcification is needed before bone can be sectioned on a routine microtome, and it destroys some staining, which is why undecalcified sections are used when mineralisation itself must be assessed
• Capillary density reflects metabolic demand — highest in myocardium and brain, lowest in tendon and cartilage